Conventional implantable cardioverter/defibrillators (ICDs) generally employ heart rate rhythm information alone to classify cardiac rhythms. U.S. Pat. No. 4,475,551 to A. A. Langer et al., entitled "Arrhythmia Detection and Defibrillation System and Method," which issued on Oct. 9, 1984, is an example of such a conventional ICD. However, there are rhythms, for example atrial fibrillation (AF) and sinus tachycardia (ST), that can cause a raised ventricular rate but that should not be treated. Thus, the use of rate only rhythm classification is not fully effective.
Similarly, rhythm discrimination in conventional ICDs is less than fully effective due to the fact that, in addition to employing rate-only rhythm classification, such ICDs employ ventricular sensing alone, rather than dual chamber sensing. In the case of ventricular-chamber-only sensing, supraventricular tachycardias are largely ignored, while any rhythm that causes a fast ventricular rate will be treated by the ICD.
Accordingly, rhythms sensed by these ICDs are thus classified as either tachycardia or not tachycardia, and therapy is applied to all tachycardia rhythms. Another example of such existing ICDs is disclosed in U.S. Pat. No. 4,830,006 to E. A. Haluska et al., entitled "Implantable Cardiac Stimulator for Detection and Treatment of Ventricular Arrhythmias" which issued on May 16, 1989. The Haluska et al. patent also employs sudden onset and rate stability in connection with its rhythm classification.
The Haluska et al. simple rate-based classifier has high sensitivity to ventricular tachycardia (VT) and ventricular fibrillation (VF), if programmed correctly. This is achieved, however, at the expense of specificity in the face of sinus tachycardia, atrial fibrillation and atrial flutter. A more advanced system should maintain the sensitivity of the simple system, while providing a substantial improvement in specificity.
The sudden onset algorithm may be useful in patients who have a sinus tachycardia/ventricular tachycardia (ST/VT) rate overlap (i.e., they have a VT which is slower than the maximum heart rate that they can achieve by exercise). There is a real concern, however, that this will compromise the sensitivity to VT as some VTs are very variable and thus may not satisfy the onset criterion. Also, it is common for VTs to be initiated by exercise. Onset has the potential to miss VTs that are initiated from an already elevated sinus rate.
Atrial fibrillation will in many cases cause a high (and usually variable) ventricular rate. A stability test may help in classifying this rhythm but, again, it risks compromising sensitivity (some VTs and most VFs display a highly unstable ventricular rate.) All of the foregoing factors lead to the conclusion that single-chamber rhythm classification has many disadvantages.
U.S. Pat. No. 4,860,749 to M. H. Lehmann, entitled "Tachycardia Detection for Automatic Implantable Cardioverter/Defibrillator With Atrial and Ventricular Sensing Capability" which issued on Aug. 29, 1989, discloses a dual-chamber hierarchical rhythm classification system which includes the following tests in connection with classifying heart rate rhythms:
1. Is the ventricular interval less than some threshold?
2. Is the ventricular interval less than the atrial interval?
3. Is the ventricular interval equal to the atrial interval?
4. Is the atrioventricular interval constant?
5. Is the ventricular interval constant?
6. Is the atrioventricular interval shorter than the predetermined sinus atrioventricular interval?
The above-cited Lehmann patent thus employs a decision tree that is based on the timings of atrial and ventricular intervals. However, it does not describe algorithms for performing any of the rhythm classification tests. Similar, earlier, disclosures of related rhythm classification systems, in which the ventricular interval is compared to a threshold, or to the atrial interval, appear in an article by R. Arzbaecher et al., entitled "Automatic Tachycardia Recognition", appearing in PACE, Vol. 7(II), pages 541-547 (May-June, 1984) and in an article by A. J. Camm et al., entitled "Tachycardia Recognition by Implantable Electronic Devices" appearing in PACE, Vol 10, pages 1175-1190 (1987)
The use of the atrioventricular (A-V) interval for rhythm classification was described in an article by J. Jenkins et al., entitled "Computer Diagnosis of Supraventricular and Ventricular Arrhythmias: A New Esophogeal Technique" appearing in Circulation, Vol 60(5), pages 977-985 (1979); in the aforementioned A. J. Camm et al. article; and in an abstract by A. D. Mercando et al., entitled "Tachycardia Differentiation Using One Atrial and Two Ventricular Electrodes", appearing in PACE, Vol. 10(II), page 415 (Abstract), (1987). Constancy of the ventricular interval was described in the aforementioned A. J. Camm et al. article and in the aforementioned U.S. Pat. No. 4,830,006 to Haluska et al.
U.S. Pat. No. 5,086,772 to D. J. Larnard et al., which issued on Feb. 11, 1992 and is entitled "Arrhythmia Control System Employing Arrhythmia Recognition Algorithm", describes a method for combining two simple morphological features with timing information to improve the rhythm discrimination process. This method considers first the rate, and uses the morphological information only in a special rate band, to classify individual heartbeats. The cardiac rhythm is then diagnosed on the basis of the classification of a number of successive heart beats. The Larnard et al. patent is assigned to the assignee of the present invention.
U.S. Pat. No. 5,000,189 to R. D. Throne et al., entitled "Method and System for Monitoring Electrocardiographic Signals and Detecting a Pathological Cardiac Arrhythmia Such as Ventricular Tachycardia" which issued on Mar. 19, 1991, is another example of the use of the shape or morphology of the intracardiac electrogram to achieve discrimination between ST and VT.
However, methods such as those used in Larnard et al. and in Throne et al. have been proven to be impractical for use in implantable cardioverter/defibrillators due to their high computational overhead. Implementation would require either excessive current drain to achieve the required computation by a conventional microprocessor, or specialized hardware that is undesirable because of cost, complexity and size.
U.S. patent application Ser. No. 07/875,161 filed Apr. 28, 1992 by D. Mason et al. and entitled "Apparatus and Method for Classifying Heart Rhythms Utilizing an Implantable Dual Chamber Cardioverter/Defibrillator", discloses a device utilizing electrograms from the atrium and the ventricle, a signal processing circuit for determining the times of atrial and ventricular events, and an algorithm for classifying the heart rhythm. This algorithm discriminates between different types of heart rhythms having overlapping ventricular rates and having similar atrial and ventricular rates, utilizing an analysis of the relationships between successive atrial and ventricular intervals (i.e., atrial-atrial, ventricular-ventricular, and atrial-ventricular). The Mason et al. application is assigned to the same assignee as the present invention.
The algorithm employed in Mason et al. classifies tachycardias, having similar atrial and ventricular rates and having ventricular-ventricular intervals which are less than a predetermined normal sinus interval, as pathological if such tachycardias:
1. have decreasing atrial-ventricular intervals which suddenly increase and then begin decreasing again; these are classified as slow dissociated ventricular tachycardias;
2. have successive increasing atrial-ventricular intervals which suddenly decrease and then begin increasing again; these are also classified as slow dissociated ventricular tachycardias;
3. have atrial-ventricular intervals which are significantly different from the atrial-ventricular intervals of a predetermined resting normal sinus rhythm; these are classified as ventricular tachycardias with a 1:1 retrograde;
4. have the difference between any two different selected atrial-ventricular intervals, in a predetermined number of consecutive atrial-ventricular intervals thereof greater than two, in excess of a predetermined value; these are classified as slow dissociated ventricular tachycardia; and
5. have their ventricular-ventricular intervals exceed a predetermined lower interval limit value and have their atrial-ventricular intervals significantly different from the atrial-ventricular intervals of a predetermined resting normal sinus rhythm; these are classified as ventricular tachycardias with a 1:1 retrograde.
Although the Mason et al. method distinguishes among physiological sinus tachycardia, pathological slow dissociated ventricular tachycardia, and pathological ventricular tachycardia with a 1:1 retrograde, based on the sequence and character of intervals between atrial and ventricular events, it cannot classify other heart rhythms, nor can it classify similar pathological rhythms that may have different manifestations than those listed above.
It is, therefore, a primary object of the present invention to provide an improved rhythm classification system for implantable dual chamber cardioverter/defibrillators.
It is a further object of the invention to provide, in an ICD, algorithms for implementing and linking together various of the aforementioned tests to produce a complete rhythm classification system.
Another object of the invention is to utilize a plurality of discriminators in combination in an ICD to provide an improved rhythm classification system.
Additional objects of the invention include the following: to significantly reduce the incidence of inappropriate therapy for ST; to reduce the need for beta-blockade as a standard management technique for patients with ST/VT rate overlap; to reduce inappropriate therapy in AF, which to a degree should remove current contra-indications of ICD therapy for patients who have AF; and to provide in an ICD the ability to confirm non-treatable tachycardias in a patient.
Further objects and advantages of the invention will become apparent as the following description proceeds.